1. Field of the Invention
The present invention relates to optical multiconductors of the silica glass type which are suited as image transmitters for various image scopes for industrial, medical and other uses, especially as image transmitters for medical image scopes.
2. Description of the Prior Art
Image scopes transmitting sharp images and having the smallest possible diameter are required for directly observing in detail portions or sites which are usually difficult to observe, such as the interior of hot blast furnaces, the interior of atomic reactors exposed to radiation, internal portions of the human body or animals, narrow or constricted inner portions of devices, etc.
Conventional image scopes have an optical multiconductor serving as an image transmitter and comprising a multiplicity of optical fibers of the multicomponent glass type. As is well known, however, multicomponent glass is low in resistance to heat and radiation and generally contains large quantities of impurities, which have heretofore presented extreme difficulties in fabricating thinner optical fibers, consequently imposing limitations on the reduction of the diameter of the optical multiconductor.
In contrast, silica glass has high resistance to heat and radiation, is excellent in drawability and therefore has the advantage that a bundle of a large required number of optical fiber preforms, each composed basically of a core and a cladding layer, can be easily drawn into a multiconductor of reduced diameter which can in no way be produced from multicomponent glass.
In the case of optical fiber preforms in which the core is made of doped silica glass, different dopants are usable in varying amounts to increase the refractive index difference between the core and the cladding layer, with the result that leakage of light can be prevented effectively even with a thin cladding layer. Accordingly, the drawing ratio can be increased to afford an optical multiconductor of decreased diameter.
For preparing such optical fiber preforms having a core of doped silica glass, there is a need to use, for example, core rods of doped silica glass, whereas such core rods heretofore used are not those prepared for producing image scopes but are only those intended for the fabrication of communication optical fibers. Moreover, the conventional core rods have a paraboric refractive index distribution that is the refractive index greatly decreases from the center of the rod toward its surface as indicated by Curve 1 in FIG. 7. Accordingly, when these core rods are used for producing an optical multiconductor, the multiconductor obtained has the problem that each fiber serving as a picture element is bright only at its central portion and greatly increases in darkness from the center toward the outer periphery. Thus, as the diameter of the multiconductor decreases, the image transmitted therethrough becomes less easy to view.